Composites and size coated glass fibers used therein

ABSTRACT

A composite which has a high flexural strength to density ratio and which is resistant to strength loss incident to exposure to water, is formed by reinforcing unsaturated thermosetting resin with glass fibers sized with selected maleic anhydride adduct of polybutadiene utilized in critical amounts.

This application is a divisional of application Ser. No. 656,211, filedOct. 1, 1984, which is a divisional of application Ser. No. 520,388,filed Aug. 4, 1983.

cl TECHNICAL FIELD

This invention relates to glass fiber reinforced plastic compositeshaving high flexural strength to density ratio and having resistance tostrength loss normally caused by exposure to moisture and/or humidity.The invention further relates to a method of making such composites.This invention further relates to size coated glass fibers for use inmaking the composites.

BACKGROUND OF THE INVENTION

There has been a continuing effort to replace metal components withlighter weight plastic components especially in motor vehicles andaircraft to reduce fuel consumption.

Glass fiber reinforced resin composites are being used for this purpose.These composites usually contain a binder to improve the interface, i.e.form a bond, between the glass fibers and the resin. The binder istypically applied as a size coating to the glass fibers. Conventionally,silane or titanate coupling agents are used for such size coating andthey provide bonding by means of Si--O--Si and Si--O--Ti linkages. Theselinkages are thermally stable but on exposure to water, e.g. moistureand/or humidity, can hydrolyze and do not reform thereby causing loss ofbonding to the reinforcement. Similarly cyclic stress, i.e. tensilestress caused by intermittent application of a load, e.g. during arevolution of the component, can cause a loss of this type of bonding.Even if an occurrence of exposure to moisture and/or humidity and/orcyclic stress does not cause failure, each exposure normally causesirreversible damage, and there is a cumulative effect. Thus, heretofore,composite material has not ordinarily replaced steel and other metals inparts, e.g. wheels for automobiles, normally encountering substantialexposure to moisture and/or humidity and/or cyclic stress.

McCombs et al. U.S. Pat. No. 4,318,960 discloses glass fiber reinforcedplastics containing glass fibers with a size coating of maleic anhydrideadduct of polybutadiene used in combination with organosilicon couplingagents. The adduct is formed in a free radical reaction in the presenceof a free radical initiator. The presence of the free radical initiatorproduces insoluble adduct. It is therefore applied to the glass fibersfrom an emulsion whereby the distribution of the sizing agent normallylacks uniformity. Furthermore, it is applied in an amount to deposit acoating of 0.1 to 15% by weight. McCombs et al. does not disclose thatits glass fiber reinforced plastics resist strength loss on exposure tomoisture and/or humidity and/or cyclic stress.

SUMMARY OF THE INVENTION

It has now been discovered that a composite having a high flexuralstrength to density ratio and which is resistant to strength lossincident to exposure to water, e.g. in the form of moisture and/orhumidity, and/or to cyclic stress is prepared by reinforcing curedunsaturated thermosetting resin with glass fibers sized with selectedsoluble maleic anhydride adduct of polybutadiene used in criticalamounts.

The composite comprises cured unsaturated thermosetting resin reinforcedby (i.e. having distributed therein) size coated glass fibers present inan amount ranging from about 20% to about 70% by weight of thecomposite.

The size coating on the glass fibers comprises maleic anhydride adductof 1,2-polybutadiene. The polybutadiene has molecular weight of morethan 10,000 and a crystallinity ranging from 0% to 50%. The adduct issoluble in organic solvent and thus can be uniformly applied as asolution. The amount of the adduct forming the size coating ranges fromabout 0.1 grams to about 6 grams per 100 grams of glass fibers. Theseamounts are critical since use of amounts of sizing less than about 0.1grams per 100 grams of glass fibers or more than about 6 grams per 100grams of glass fibers results in an unacceptably increased tendencytoward failure at the interface.

The composites herein are made by a process comprising the steps of:

(a) forming the maleic anhydride adduct of 1,2-polybutadiene by reactingmaleic anhydride with 1,2-polybutadiene having a number averagemolecular weight of more than 10,000 and a crystallinity ranging from 0%to 50%, said reaction being carried out under an inert atmosphere at atemperature ranging from about 150° C. to about 260° C. and in theabsence of a free radical initiator, the weight ratio of polybutadieneto maleic anhydride ranging from about 100:1 to about 1.5:1;

(b) dissolving the adduct in powder form in an organic solvent;

(c) coating glass fibers with the solution produced in step (b)utilizing from about 0.1 grams to about 6 grams of adduct per hundredgrams of glass fibers;

(d) evaporating the solvent to provide glass fibers size coated withadduct;

(e) admixing the size coated glass fibers with unsaturated thermosettingresin and curing agent, the size coated glass fibers being present in anamount ranging from about 20% to about 70% by weight of the admixture;

(f) curing to form a composite.

The step of forming the adduct under an inert atmosphere and in theabsence of a free radical initiator, i.e. step (a), is critical. Theseconditions curb cross linking so that the adduct is readily dissolved instep (b) whereby it is easily and uniformly applied to provide aninterface bond resistant to hydrolysis whereby the composite isresistant to strength loss normally caused by exposure to water.

DETAILED DESCRIPTION

The composites herein normally have a flexural strength, i.e. a breakstrength, ranging from about 25,000 to about 35,000 psi even afterprolonged exposure to water. Such flexural strength approaches that ofsteel. The density of the composites is on the order of 1 to 2 grams percubic centimeter while that of steel is around 8. Thus, the compositesherein have a greater flexural strength to density ratio than steel sothat thickness of composite parts can be increased compared tocorresponding steel parts to increase strength to that of steel whilestill obtaining a weight decrease.

The composites herein normally have a flexural modulus, i.e. an initialresistance to bending, ranging from about 10⁶ to about 10⁷ psi comparedto 2.8×10⁷ psi for a piece of steel of the same thickness.

The composites herein also have excellent elongation properties andenergy at break properties (e.g. 1.85-3.0% elongation and 400-600 psifor energy at break).

Not only do the composites herein resist loss of flexural strength onexposure to water, they also resist loss in respect to the otheraforementioned strength properties (i.e. flexural modulus, % elongation,energy at break). The composites herein also resist loss in respect tothe aforementioned strength properties on exposure to cyclic stress.

The strength figures herein are those determined according to ASTMtesting procedure D-790.

The composites herein are excellent substitutes for metal in thefabrication of wheels for automobiles and of aircraft components.

In describing the composites and their size coated glass fiberreinforcing, we turn firstly to the particular maleicanhydride-polybutadiene adduct used in critical amounts to provide thesize coating whereby the unique strength properties herein are obtained.

Some adducts of this type are described in Imaizumi et al. U.S. Pat. No.4,082,817.

The polybutadiene constituent is referred to herein as1,2-polybutadiene. The terminology 1,2-polybutadiene is used herein toinclude polybutadienes containing about 85% or more 1,2-polybutadienewith a remainder being 1,4-polybutadiene.

The 1,2-polybutadiene ordinarily can have a number average molecularweight up to about 200,000. The term "number average molecular weight"is used herein to mean a number average molecular weight determined fromthe intrinsic viscosity ([η]) of the polymer using the followingequation:

    [η]=9.1×10.sup.-5 ×Mn.sup.0.8

wherein [η]is an intrinsic viscosity measured in toluene at 30° C. andMn is a number average molecular weight [see Y. Takeuchi, et al., "A NewThermoplastic Syndiotactic 1,2-Polybutadiene" in Coatings and PlasticsPreprints, Vol. 34, No. 1, p. 123 (American Chemical Society) (April1974)].

The 1,2-polybutadiene utilized is preferably amorphous.

The reaction of the 1,2-polybutadiene and maleic anhydride is preferablycarried out with a weight ratio of 1,2-polybutadiene to maleic anhydrideranging from about 3:1 to about 2:1. The weight ratio used determinesthe number of milliequivalents of anhydride per gram of adduct.

The time of the reaction is temperature dependent with shorter timesbeing utilized for higher temperatures. For example, times on the orderof one hour are useful at the 260° C. temperature while reaction at 150°C. can take up to a day or more.

The reaction can be carried out either with or without solvent. When asolvent is utilized it preferably is a halogenated aromatic hydrocarbonsuch as, for example, monochlorobenzene, o-dichlorobenzene, orα-chloroaphthalene.

It is critical herein that the reaction be carried out under an inertatmosphere and in the absence of free radical initiators. The reactionis carried out in this way so as not to foster free radical formation.This is important because free radical formation fosters cross linkingwhich in turn causes the adduct to be difficult to dissolve. A solubleadduct is critical herein so the adduct can be readily applied andevenly distributed from solution whereby the strength retentionproperties are obtained when the adduct is applied in the criticalamounts stated herein.

Carrying out the reaction under an inert atmosphere involves excludingoxygen, e.g. by application of vacuum and/or purging with nitrogen orother unreactive gas and reacting under vacuum or nitrogen or otherinert gas preferably in a system closed to oxygen entry.

It is highly preferred that the reaction be carried out in the presenceof antioxidant to hinder the tendency of any residual oxygen which ispresent to cause free radical formation. Suitable antioxidants include,for example, hindered phenolics (e.g. that sold under the name Irgonox),dibutylparacresol, dibutylphenylcresol (e.g. that sold under the nameEastzone DB), and octyl substituted paraphenylenediamine (e.g. that soldunder the name Eastzone 31).

The reaction ordinarily produces a brittle mass of reaction product. Thebrittle mass is ordinarily size reduced to form powder for ease ofdissolving; this is readily carried out with impacting means.

Turning now to the dissolving step, the organic solvent should be onethat does not react in non-reversible fashion with the anhydride. Apreferred organic solvent for use herein in dissolving the powderedadduct is acetone. Other useful solvents include chloroform, ketones,ethers, tetrahydrofuran, dioxane, and aqueous tertiary amines. Alcoholsare not useful solvents since they react with the anhydride in anon-reversible fashion.

In dissolving the adduct in the solvent, use of a small amount ofsolvent results in a viscous solution which is hard to handle while useof a large amount of solvent increases the evaporation load. Use ofsolvent to adduct ratio (volume basis) ranging from about 5:1 to about15:1 provides good results with a ratio of about 8:1 to 12:1 beingpreferred.

The adduct solution is readily applied to coat glass fibers by passingthe fibers through the solution as continuous filaments as they areproduced. The adduct solution is also readily applied by admixing adductsolution with fibers already cut to length.

Evaporation of the solvent to provide glass fibers size coated withadduct is readily carried out by conventional drying methods, e.g. byexposing the solution coated fibers to vacuum under a hood or in avacuum oven.

Preferably, the amount of adduct used to coat the glass fibers andforming the size coating ranges from about 1 gram to about three gramsper hundred grams of glass fibers.

The glass fibers utilized are preferably in the form of bundles of glassfibers ranging from 0.5 to 1.5 inches in length.

The glass fibers utilized can be free from organosilane coupling agentsbut the benefit of the invention of resistance to strength loss normallycaused by exposure to water is obtained despite the presence of such.This is important because some glass fibers are available from themanufacturer already treated with silane coupling agents, e.g. 0.5-2% byweight of the fibers of silane based coupling agents.

Turning now to the step where the size coated glass fibers are admixedwith unsaturated thermosetting resin and curing agent, this is readilyaccomplished, for example, by placing the size coated glass fibers in amold or other container and pouring therein the resin plus curing agentand any other additives. Preferably pressure is applied in the moldingto close any air gaps between the fibers.

Preferably, the size coated glass fibers are present in the admixture inan amount ranging from about 30% to about 50% by weight of the admixtureso as to be present in these amounts by weight of the composition.

The unsaturated thermosetting resin can be, for example, a vinyl esterresin having terminal unsaturation, for example, a resin having astructure as follows: ##STR1## Alternatively, the unsaturated resin canbe unsaturated polyester resin; these resins typically have theirunsaturation contributed by interesterified unsaturated dicarboxylicacids, such as maleic acid. The unsaturated thermosetting resin can alsobe, for example, a high vinyl polybutadiene or a polar-nonpolar blockpolymer (e.g. where nylon or polyimide or polyurea or polyurethane formsthe polar block and 1,2-polybutadiene forms the nonpolar block).

Preferably, the unsaturated thermosetting resin is a vinyl ester. A verysuitable one in styrene is sold under the name Derakane 790 by DowChemical; Derakane 790 is described by the manufacturer as typicallyhaving the following properties: an acid number of 25; a kinematicviscosity, cks at 25° C. of 1200; a specific gravity of 1.03; and an SPIgel time in minutes at 180° F. of 30.

When a vinyl ester resin is utilized, a thickener is preferably includedwith it as is conventional in vinyl ester curing. Thus, in this case,the size coated glass fibers are admixed with vinyl ester resin,thickener and curing agent. The thickener can be, for example, magnesiumoxide or calcium oxide. Thickening is carried out simply by includingthe thickener in the admixture and letting the admixture sit in the moldor container where the admixture is formed. Thickening is readilycarried out, for example, in a day or two.

What curing system is utilized depends on what unsaturated thermosettingresin is utilized, and the appropriate curing systems are well known inthe art.

For the vinyl ester resins preferred herein and for the unsaturatedpolyester resins, curing agent can be, for example, t-butyl perbenzoate,benzoyl peroxide, methyl ethyl ketone peroxide, or cyclohexanoneperoxide. Curing vinyl ester resin with such curing agent can be carriedout, for example, in the mold at a temperature ranging from about 60° C.to about 190° C. over a time period ranging from about 1 minute to about4 hours with the time and temperature being selected based on theparticular curing agent selected. With t-butyl perbenzoate, curing overa 10 minute period at 150° C. provides very satisfactory results.

Curing agents for the unsaturated thermosetting resins includingpolybutadiene include, for example, di-t-butylperoxide,α,α'-bis(t-butylperoxy)diisopropyl benzene,2,5-dimethyl-2,5-(t-butylperoxy)hexane, and dicumyl peroxide.

The invention is illustrated in the following specific example.

EXAMPLE

Maleic anhydride adduct of 1,2-polybutadiene was prepared as follows:The materials were added in the proportion of 2 parts by weightamorphous polybutadiene to 1 part by weight maleic anhydride into twoinch diameter tubes with the bottom sealed. Included in the materialsadded was antioxidant consisting by weight of 0.1 partdibutylphenylcresol (Eastzone DB) per 100 parts of polybutadiene and0.01 part of octyl substituted paraphenylenediamine (Eastzone 31) per100 parts of polybutadiene. The tubes were evacuated, then nitrogenpurged, then sealed under vacuum, then heated in an oven at 175° C. for20 hours. The reaction produced a brittle mass of adduct containing 2.8milliequivalents of anhydride per gram.

The brittle masses of adduct were broken up using a hammer to providepowder.

The powder was dissolved in acetone to produce an adduct solution.

The adduct solution was applied to bundles of 1 inch glass fibers(formed from a continuous strand cut every inch with each filament inthe strand having a diameter of about 10 microns with about 1000filaments constituting the strand). The fibers utilized were sold underthe designation 4225 by PPG Industries and are described by themanufacturer as having been treated with silane coupling agent that iscompatible with peroxide cured unsaturated resins.

The solution application was carried out as follows: The solution and 60grams of fiber bundles were placed in a 6×6×3 aluminum tray in relativeamounts as set forth in the table below and drying to evaporate thesolvent was carried out by drawing a vacuum under a hood for 16 hours,then heating at 60° C. in a vacuum oven for two hours.

Composite formation was carried out by forming an admixture of Derakane790 (vinyl ester resin in styrene) magnesium oxide thickening agent andt-butyl perbenzoate curing agent and introducing 60 grams of this intothe aluminum tray containing the adduct coated glass fibers. Thickeningwas allowed to occur for two days. Then curing was carried out for 10minutes at 150° C.

Batches were made up with 1 gram of adduct per 60 grams of glass fibers(Run 1), no grams of adduct per 60 grams of glass fibers (Run 2), and 5grams of adduct per 60 grams of glass fibers (Run 3). Run 1 is withinthe scope of the invention while Runs 2 and 3 are not. For each runtesting was carried out on composite samples which were not exposed towater and also on composite samples which were submerged in boilingwater for four weeks and tested after drying. Testing was carried out inaccordance with ASTM testing procedure D-790.

The results are presented in the Table below where "No Water" means notexposed to water and where "Water Treated" means the aforedescribedboiling water treatment.

    ______________________________________                                        COMPARISON TESTING                                                                           Run 1   Run 2   Run 3                                          ______________________________________                                        Flexural Strength × 10.sup.-3                                           (psi)                                                                         No Water         32.3      37.5    30.9                                       Water Treated    29.9      23.4    17.7                                       Flexural Modulus × 10.sup.-6                                            No Water         1.90      1.63    2.01                                       Water Treated    1.84      1.71    1.22                                       % Elongation                                                                  No Water         2.42      2.92    2.03                                       Water Treated    2.03      1.61    2.30                                       Energy at Break                                                               (psi)                                                                         No Water         479       608     357                                        Water Treated    496       207     243                                        ______________________________________                                    

As can be seen from the above, the invention (Run 1) provides the bestoverall results for the water-treated composite when compared to no sizecoating (Run 2) and too much size coating (Run 3).

Similar results of good strength properties retention after exposure towater are obtained when other vinyl ester resins are used in place ofDerakane 790 or when other unsaturated thermosetting resins, e.g. highvinyl polybutadienes, are used in place of the Derakane, or when glassfibers free of silane are utilized.

While the foregoing describes certain preferred embodiments of theinvention, modifications will be readily apparent to those skilled inthe art. Thus, the scope of the invention is intended to be defined bythe following claims.

I claim:
 1. Glass fiber reinforced composite having high flexuralstrength to density ratio and resistance to strength loss incident toexposure to water, said composite comprising cured unsaturatedthermosetting resin reinforced by glass fibers having thereon a sizecoating comprising maleic anhydride adduct of 1,2-polybutadiene having anumber average molecular weight of more than 10,000 and a crystallinityranging from 0% to 50%, said adduct being soluble in an organic solvent,the amount of said adduct forming the size coating ranging from about0.1 grams to about 6 grams per 100 grams of glass fibers, said sizecoated glass fibers being present in an amount ranging from about 20% toabout 70% by weight of said composite.
 2. Glass fiber reinforcedcomposite as recited in claim 1, wherein said size coated glass fibersare present in an amount ranging from about 30% to about 50% by weightof the composite.
 3. Glass fiber reinforced composite as recited inclaim 2, wherein the amount of adduct in the size coating ranges fromabout 1 gram to about 3 grams per hundred grams of glass fibers. 4.Glass fiber reinforced composite as recited in claim 3, wherein theunsaturated thermosetting resin is a vinyl ester resin.
 5. Glass fiberreinforced composite as recited in claim 4, wherein the glass fibers arein the form of bundles of glass fibers ranging from 0.5 to 1.5 inches inlength.
 6. Glass fibers reinforced composite as recited in claim 5,wherein the polybutadiene used in the size coating is amorphous and theweight ratio of polybutadiene to maleic anhydride used to form theadduct ranges from about 3:1 to about 2:1.